Forensic science experts from the Metropolitan police have been helping Portuguese police investigating the disappearance of Madeleine McCann to put together the sort of evidence provided by mobile phone records that solved the Soham murders of 10-year-olds Holly Wells and Jessica Chapman.

At least one of the British investigators seen at the holiday complex in the resort town of Praia da Luz, where four-year-old Madeleine went missing, has been identified as the Metropolitan police's top digital analysis expert.

Experts said the investigator - who is not a police officer - would be mapping how different mobile phone masts served various parts of the Praia da Luz resort and nearby areas.

"You would need to go to Praia da Luz first to see which networks provide a service there," said David Bristowe, the mobile phone expert whose evidence helped solve the Soham murders in 2002.

This could provide Portuguese police with a valuable way of checking the alibis of any suspects who used mobile telephones that night, he said. "Generally speaking, mobile phone evidence is better for showing where someone is not, rather than where somebody is."

The information would be compared with data stored by Portuguese mobile phone companies showing calls that went through local masts that night. Records could also provide a list of anyone who used a mobile phone in Praia da Luz around the time Madeleine disappeared, helping police to identify strangers who were in town that night.

Portuguese police have this week gone door to door in Praia da Luz, asking people to say where they were on May 3 and to give their mobile phone numbers. "What they are often looking for are inconsistencies," said Professor Jan Stuart, a leading British forensic engineering expert.

Mobile phone evidence exposed holes in the alibi of Ian Huntley, who murdered Holly and Jessica, and showed that the girls had been in or very close to his house.

Chief Inspector Olegario Sousa, of the Portuguese police in the Algarve, said yesterday that British police had taken the initiative in sending out experts.

"The technicians came to Portugal after a number of requests from our British counterparts to be allowed to conduct their own investigations," he said. The technicians have since returned to Britain.

Police have one formal suspect in the case, British expatriate Robert Murat, 33, who lives 150 metres from the Ocean Club complex where Madeleine went missing. He has protested his innocence and reportedly told police he was at home with his mother, Jenny, that evening.

DNA analysis of evidence found in the McCanns' holiday flat shows that someone from outside the family had been there, Portugal's 24 Horas newspaper reported yesterday. The newspaper, which quoted unnamed sources at forensic science laboratories, said the DNA evidence did not match that of any suspects and was being sent on to investigators yesterday.

"I cannot deny the reports," said Mr Sousa.

As relatives of Madeleine yesterday released 400 yellow balloons at a Glasgow primary school, Madeleine's parents took their campaign to Spain, calling for help from both Spanish police and the general public, amid concern that Madeleine may have been taken out of Portugal.

Her father, Gerry McCann, said they hoped Madeleine, who will have been missing for a month tomorrow, was being held by someone who was looking after her. "We also pray that the person who has her gives her up voluntarily and drops her at a safe place."